1. Field of the Invention
The present general inventive concept relates to a wireless communication system, and more particularly, to a method and apparatus to improve stability and efficiency of data transmission over a wireless network.
2. Description of the Related Art
Recently, as wireless communication networks have been widely used, and a large-volume of multimedia data is transmitted through the wireless communication networks, there is a need to develop a better and more efficient method of transmitting data over the wireless communication networks. Due to existing characteristics of conventional wireless networks, which are accessed a large number of devices, operating characteristics of the convention wireless networks deteriorate with increased data traffic, which often results in data collision or loss during transmission. In order to prevent the data collision or loss and to receive/transmit data reliably, a distributed coordination function (DCF) on a competitive basis and a point coordination function (PCF) on a non-competitive basis have been employed in wireless local area networks (LAN). In wireless personal area networks (PAN), channel time allocation has been employed.
Although these methods employed in the wireless networks reduce the data collision to some degree and facilitate stable data transmission, there is still a high chance of the data collision during transmission, compared to wired networks, due to various factors, such as multi-path fading and interference that affect stable data transmission. In addition, as the number of wireless devices that access a wireless network increases, the data collision and loss increases.
The data collision and loss result in re-transmission of the lost data which adversely affects the throughput of a wireless network. In particular, for A/V data which requires a high quality of service (QoS), it is critical to have a sufficient bandwidth to minimize the number of re-transmissions.
FIG. 1 is a view illustrating a method of transmitting data over a conventional wireless network. When a device in the wireless network, that is, a transmitting device or source device, sends a data frame to another device in the wireless network, that is, a receiving device or destination device, the transmitting device generates a media access control (MAC) layer (MAC header (MAC HDR) or a physical (PHY) layer header (PHY HDR), appends the generated MAC HDR and PHY HDR to the data to form the data frame, and transmits the data frame.
The receiving device that receives the data is able to check for errors via a frame check sequence (FCS), and sends the received data to an upper layer if no error has been found.
An IEEE 802.11 protocol assures a short inter-frame space (SIFS), i.e., a minimum space between frames. The SIFS includes a delay in the PHY layer, and a time required to process the frame and a turnaround time of a receiving/transmitting device (RX/TX) in the MAC layer. That is, the SIFS is a time indicated by the protocol to receive a single frame from a single device, process the frame, and generate and send a response frame. Therefore, if the transmitting device transmits numerous frames to the receiving device, time is needed to transmit the frames as well as to transmit corresponding SIFS and response frames.
FIG. 2 is a view illustrating a conventional method of handling errors generated during transmission between wireless devices. Referring to FIG. 2,the conventional method includes handling a transmission failure between a transmitting (source) device and a receiving (destination) device in a wireless network environment, when the transmission failure has been caused by various reasons or when an error occurs in the transmitted frame.
In the transmitting device, a counter “acknowledge (ACK) Timeout ” starts countdown from the moment of transmitting the frame. When the transmitting device fails to receive an ACK frame (hereinafter, referred to as “ACK”) from the receiving device when the counter reaches zero, the transmitting device determines a transmission failure and re-transmits the frame. The frames that have not been transmitted or the frames with errors are illustrated as dotted lines in FIG. 2.
The number of re-transmissions depends on a value of “retry limit” set in the transmitting device. If the transmitting device fails to re-transmits by the retry limit, the transmitting device skips the frame. As illustrated in FIG. 2, the frame with data 2 is skipped. The receiving device checks out the received frame to find MAC FCS and if no error is found, the receiving device sends the data appended to the received frame to an upper layer and sends a response frame to the received frame, that is, an ACK, to the transmitting device. If an error is found, the receiving device stop forming and sending the corresponding frame.
As illustrated in FIG. 3, the transmitting device regards all times except for a time required to transmit a frame, as a wasted time. the wasted time includes a time between frames being transmitted and a time required for a PHY header (hereinafter, referred to as “header overhead”) and a MAC header to be appended to each frame, which causes inefficiency of an entire wireless network. When a 188-byte MPEG-2 transport stream is transmitted, a relatively large header overhead is required due to a high frequency of transmission. This is because an IEEE 802.11 standard requires an ACK for each frame being transmitted.
According to the standard, a size of the MAC header appended to the data frame is 30 bytes, and the time required for the PHY header is 20 μs. Therefore, the header overhead becomes more significant and larger with respect to the data actually being transmitted, thereby decreasing an efficiency of a wireless network.